Karyotyping is a genetic test that examines the chromosomes inside your cells, checking whether you have the right number (46 in humans), the right sizes, and the right structures. By laying out a photograph of all your chromosomes in ordered pairs, the test can reveal missing, extra, or rearranged genetic material that causes conditions like Down syndrome, Turner syndrome, and certain cancers.
What Chromosomes Tell Us
Every cell in your body contains 23 pairs of chromosomes, for a total of 46. These chromosomes are tightly coiled packages of DNA that carry all the instructions for how your body grows and functions. Twenty-two of those pairs are numbered 1 through 22, and the final pair determines biological sex: two X chromosomes in females, one X and one Y in males.
Problems arise when chromosomes are added, lost, or structurally altered. Having three copies of a chromosome instead of two is called trisomy. Having only one copy instead of two is called monosomy. Pieces of chromosomes can also break off, flip around, or swap places with segments of other chromosomes. A karyotype test is designed to catch all of these large-scale changes in a single snapshot.
How the Test Works
Karyotyping can be performed on almost any tissue that contains dividing cells. The most common sample types are blood, bone marrow, amniotic fluid (collected through amniocentesis during pregnancy), and placental tissue. A blood draw is the simplest route for most non-pregnant patients.
Once collected, the cells are placed in a nutrient-rich dish and allowed to grow in a lab. At a critical point, technicians add a chemical that freezes the cells mid-division, right at the stage when chromosomes are most condensed and visible. The cells are then placed on a glass slide, treated with a special stain called Giemsa, and examined under a microscope.
The Giemsa stain, developed in the 1970s, creates a distinctive pattern of dark and light bands along each chromosome. Because every chromosome has its own unique banding pattern, a trained cytogeneticist can identify each one, line them up in numbered pairs, and spot anything out of place. This technique is known as G-banding and remains the standard method for routine chromosome analysis.
What Karyotyping Can Detect
The test picks up two broad categories of abnormalities: changes in chromosome number and changes in chromosome structure.
- Numerical changes. Down syndrome (trisomy 21) results from an extra copy of chromosome 21. Turner syndrome occurs when a female has only one X chromosome instead of two. Other trisomies, like those involving chromosomes 13 or 18, cause severe developmental problems detectable before or after birth.
- Structural changes. In a translocation, a segment of one chromosome breaks off and reattaches to a different chromosome. In an inversion, a segment flips orientation within the same chromosome. Deletions involve a missing chunk of a chromosome, while duplications mean a section has been copied. Ring chromosomes, where the ends of a chromosome fuse into a loop, are also visible on a karyotype.
Some structural rearrangements are “balanced,” meaning no genetic material is actually gained or lost. A person carrying a balanced translocation may be perfectly healthy but could pass an unbalanced version to their children, leading to miscarriages or birth defects.
Why Doctors Order the Test
Karyotyping serves different purposes depending on the clinical situation.
In prenatal care, it is ordered when an ultrasound shows unexpected findings, when blood screening results are abnormal, when a noninvasive prenatal screening test flags a concern, or when the mother is over 35. It can also be recommended for couples with a history of two or more miscarriages, stillbirths, or known chromosome conditions in the family.
In oncology and blood disorders, karyotyping helps diagnose and classify leukemias and other cancers. Cancer cells often carry characteristic chromosome rearrangements that influence both the expected outcome and the choice of treatment. Repeat karyotyping during treatment can show whether the abnormal cells are being eliminated.
Outside of pregnancy and cancer, the test is used to investigate unexplained developmental delays, multiple birth defects, ambiguous physical sex characteristics, or infertility in both men and women.
What Karyotyping Cannot See
Standard karyotyping works at a resolution of roughly 5 to 10 megabases, which means it can only detect changes involving at least 5 to 10 million base pairs of DNA. That sounds like a lot, but it is actually a small fraction of the total genome, and many disease-causing changes are far smaller. Single-gene mutations, tiny deletions, or duplications below that threshold will not show up on a karyotype.
For those smaller changes, doctors turn to higher-resolution tools like chromosomal microarray analysis, which can detect copy number changes down to a few hundred thousand base pairs, or targeted molecular tests that zero in on individual genes. Karyotyping remains valuable because it provides a whole-genome overview in one test, catching large rearrangements and balanced translocations that some newer technologies miss.
Reading a Karyotype Report
Results are written in a standardized shorthand. A normal male karyotype is reported as 46,XY. A normal female is 46,XX. The first number indicates the total chromosome count, and the letters specify the sex chromosomes.
An abnormal result adds extra notation. For example, 47,XY,+21 describes a male with 47 chromosomes, the extra one being a third copy of chromosome 21 (Down syndrome). 45,X describes a female with only 45 chromosomes due to a missing X (Turner syndrome). Structural changes are noted with abbreviations for the type of rearrangement and the chromosome regions involved.
What to Expect as a Patient
If your sample comes from a simple blood draw, the experience is no different from any routine lab test. Prenatal karyotyping requires an amniocentesis or a sample of placental tissue, which are more involved procedures your doctor will walk you through. Bone marrow karyotyping requires a biopsy, typically done under local anesthesia.
Because the cells need time to grow in the lab before they can be analyzed, karyotype results are not immediate. Turnaround times vary by laboratory and sample type, but most results take one to three weeks. Your healthcare provider will explain the findings and, if an abnormality is found, discuss what it means for your health or pregnancy and what steps come next.